This invention relates to a multiple transmission-type photoelectric sensor for sensing a plurality of objects at one time, a single transmission-type photoelectric sensor capable of being applied particularly to a multiple transmission-type photoelectric sensor, and a photoelectric sensing method.
Available as one example of a multiple transmission-type photoelectric sensor is a wafer sensor used in a semiconductor-wafer manufacturing process to check semiconductor wafers to determine whether or not they are present, monitor the wafers and verify the number thereof on a lot-by-lot basis.
Sensors described in the specifications of Japanese Patent Publication No. 6-11070 and Japanese Utility Model Application Laid-Open No. 5-66987 are examples of wafer sensors. The wafer sensors described in this literature include a light-projecting element and a photoreceptor element forming a pair and disposed so as to oppose each other. A multiplicity of these light-projecting and photoreceptor elements are inserted between wafers held in a wafer cassette at regular intervals in such a manner that the wafers will be sandwiched by the pairs of light-projecting and photoreceptor elements. A pair of the light-projecting and photoreceptor elements sandwiching a wafer constructs a transmission-type photoelectric sensor. Light projected from the light-projecting element is blocked if a wafer is present but is received by the corresponding photoreceptor element in the absence of a wafer.
Since a semiconductor wafer is opaque, its absence or presence can be sensed on the basis of whether or not the projected light is blocked, as described above. In recent years, transparent or semi-transparent wafers that rely on quartz glass, sapphire glass, liquid-crystal glass and silicon-carbide glass have come to be used for a variety of applications. These transparent or semi-transparent wafers cannot be sensed by, or are difficult to sense by, the above-mentioned wafer sensor. The reason for this is that a transparent wafer transmits most of the projected light from the light-projecting element so that the projected light reaches the photoreceptor element with little attenuation. The difference in amount of light received by the photoreceptor element when a transparent wafer is and is not present is very small and is difficult to identify. In addition, extraneous light and a change in the characteristics of the light-projecting and photoreceptor elements due to temperature are not negligible.
A wafer sensor that is applicable to both transparent and opaque wafers is illustrated in the specification of Japanese Patent Application Laid-Open No. 6-77307. This wafer sensor includes a first element having a light-emitting surface and a photoreceptor surface, and a second element having a photoreceptor surface, the elements being disposed so as to oppose each other. A wafer is inserted between the first and second elements. For a wafer that is opaque, light projected from the first element is blocked by the wafer if the wafer is present. If the wafer is absent, the projected light is received by the second element. In the case of a transparent wafer, light projected from the first element and reflected by the wafer (if the wafer is present) is received by the photoreceptor surface of the first element. If the wafer is absent, light does not impinge upon the photoreceptor surface of the first element. This is premised on the fact that whether the wafer is a transparent wafer or an opaque wafer is known beforehand. The photoreception signal of the first element and the photoreception signal of the second element are switched, depending upon the type of wafer, before being applied to a discriminating circuit.
This wafer sensor requires the first element having the light-emitting and photoreceptor surfaces. In the case of a transparent wafer, the sensor receives the light reflected from the wafer and therefore is readily influenced by the surface of the wafer. The wafer type, i.e., transparent or opaque, must be known in advance.
In the wafer sensors of all of the above-mentioned types, a light-projecting (light-emitting) element and a photoreceptor element must be inserted in the gaps between wafers. There has been a tendency in recent years for the gaps between the multiplicity of wafers held in the wafer cassette to be made smaller. There is a limit upon the extent to which the thickness of the light-projecting and photoreceptor elements and the thickness of the members for holding these elements can be reduced.
There are occasions where the wafers in a semiconductor process become charged with static electricity. If this static electricity discharges through electrically conductive portions of the light-projecting and photoreceptor elements inserted between the wafers, there is the danger that this may lead to erroneous detection and destruction of the light-projecting and photoreceptor elements.
An object of the present invention is to provide a transmission-type photoelectric sensor, multiple transmission-type photoelectric sensor and photoelectric sensing method capable of sensing both opaque and transparent bodies (inclusive of semi-transparent bodies).
Another object of the present invention is to provide a structure whereby a sensing portion inserted between objects to be sensed can be made as thin as possible.
A further object of the present invention is to provide a structure whereby the influence of static electricity that has charged an object to be sensed can be made as small as possible.
Still another object of the present invention is to provide a structure whereby the number of light-projecting and photoreceptor elements can be made as small as possible.
A multiple transmission-type photoelectric sensor according to the present invention is defined as follows when expressed all-inclusively: Specifically, a multiple transmission-type photoelectric sensor according to the present invention has a plurality of sensing arms provided in spaced-apart relation on a sensor case so as to extend outwardly of the case, and a plurality of light-projecting elements and a plurality of photoreceptor elements provided inside the sensor case, one light-projecting element, one photoreceptor element or one pair of the light-projecting and photoreceptor elements corresponding to each sensing arm, a distal end of each sensing arm being provided with at least one of a first deflecting member for directing projected light from the corresponding light-projecting element toward a neighboring sensing arm and a second deflecting member for directing projected light from the neighboring sensing arm toward the corresponding photoreceptor element.
In one embodiment, one pair of the light-projecting and photoreceptor elements corresponds to each sensing arm, and each sensing arm is provided with the first deflecting member and the second deflecting member. In another embodiment, one light-projecting element or one photoreceptor element corresponds to each sensing arm, the distal end of the sensing arm that corresponds to the light-projecting element is provided with the first deflecting member, and the distal end of the sensing arm that corresponds to the photoreceptor element is provided with the second deflecting member.
In any case, the space between two neighboring sensing arms is a sensing area, projected light from the light-projecting element of one sensing arm reaches the neighboring other sensing arm by traversing the sensing area at least one time (two or more times depending upon the mode) and is received by the photoreceptor element of the one sensing arm or of the other sensing arm. By subjecting the photoreception signal of the photoreceptor element to level discrimination, at least the absence or presence of an object in the sensing area is determined.
In accordance with the present invention, a plurality of sensing arms are provided. As a result, a plurality of sensing areas are established and sensing operations can be performed simultaneously in these plurality of sensing areas. The distal end of each sensing arm need only be provided with a deflecting member (e.g., a reflecting surface, a prism, etc.); provision of a light-projecting element and photoreceptor element is not necessary. Accordingly, it is possible to reduce the thickness of the sensing arm so that the sensing arm can be applied to a narrow sensing area. In a case where the photoelectric sensor according to the present invention is used in sensing a wafer or the like, the fact that the distance between the wafer and the light-projecting and photoreceptor elements is great (a long distance can be set) means that even if the wafer becomes charged with static electricity, it is possible to prevent the light-projecting and photoreceptor elements and the sensing circuit from being adversely affected by this static electricity.
In a preferred embodiment, each sensing arm is provided with a third deflecting member for returning the projected light from the neighboring sensing arm to this neighboring sensing arm.
The projected light traverses the space (the sensing area) between neighboring sensing arms at least two times. Even if an object to be sensed is transparent or semi-transparent, the projected light passes through the object at least twice, as a result of which the amount of attenuation increases to make possible reliable detection of a transparent or semi-transparent object.
In another preferred embodiment, the first deflecting member directs the optical path of the projected light obliquely with respect to a direction in which the sensing arms are arrayed, and the second deflecting member directs the light, which has advanced obliquely with respect to the direction in which the sensing arms are arrayed, toward the photoreceptor element.
The projected light impinges upon the surface of the object within the sensing area obliquely and passes through the object obliquely. Since loss equivalent to the amount of reflection of the light that impinges obliquely upon the surface of the object is great, it is possible to sense the object reliably regardless of whether it is transparent or semi-transparent.
When the projected light passes through the object obliquely, the optic axis is displaced. It is preferred that the front side of the photoreceptor element be provided with a slit which limits the incident light in such a manner that the displacement of the optic axis can be sensed more noticeably. The amount of light that impinges upon the photoreceptor element varies greatly depending upon whether or not an object is present (even if the object is a transparent object) and the type of object, as a result of which more certain detection becomes possible.
The displacement of the optic axis can be sensed by a position sensing device and the absence or presence of an object and the type thereof can be determined based upon an output representing the position sensed by the position sensing device.
If the output representing the sensed position and a photoreception output are obtained from the position sensing device, these outputs are each discriminated by a predetermined threshold value and the results of discrimination are subjected to a logic operation, it is possible to sense an object much more reliably (as well as the type of object when necessary).
In yet another preferred embodiment, the first deflecting member splits the projected light from the light-projecting element into two portions and directs these two portions toward the neighboring sensing arms on both sides.
Since the projected light from one light-projecting element is split into two portions and propagates into two sensing areas, the number of light-projecting and photoreceptor elements can be reduced.
In order that objects to be sensed in a plurality of sensing areas may be sensed in time-shared fashion, there are provided drive means for driving a plurality of light-projecting elements sequentially at predetermined time intervals, and means for fetching, in sync with driving of the light-projecting element, a photoreception signal of a prescribed photoreceptor element which receives projected light from a light-projecting element that is driven. There is further provided decision means having at least one threshold value for discriminating an output signal of a photoreceptor element based upon this threshold value, thereby outputting a detection signal indicative of an object to be sensed.
There are a variety of threshold values. One is for distinguishing between absence of a transparent body and presence of a transparent body. A second is for distinguishing between absence of a semi-transparent body and presence of a semi-transparent body. A third is for distinguishing between a transparent body and a semi-transparent body. A fourth is for distinguishing between a transparent body and an opaque body. A fifth is for distinguishing between a semi-transparent body and an opaque body. A sixth is for distinguishing between absence or presence of an object. These threshold values may be combined appropriately.
In one embodiment, the sensing arms are provided with at least one of a first light guide for guiding projected light from a light-projecting element to the first deflecting member and a second light guide for guiding light from the second deflecting member to a photoreceptor element. Light can be guided reliably in the sensing arm and the effects of extraneous light can be reduced.
Preferably, the case is provided with at least one of a shield member covering the light-projecting element, a shield member covering the photoreceptor element, a shield member covering the space between the sensing arms, a shield member covering the entirety of the plurality of photoreceptor elements, and a shield member covering a circuit board on which the sensing circuit is provided. This furnishes a greatly enhanced static-electricity countermeasure.
A first type of single transmission-type photoelectric sensor (which can also be used in multiple-type configuration as a matter of course) according to the present invention has a light-projecting element and a photoreceptor element for receiving light that has traversed a sensing area upon being emitted by the light-projecting element, characterized by having: a first deflecting member for bending, approximately at right angles, projected light from the light-projecting element to direct the projected light toward the sensing area; a second deflecting member for bending, approximately at right angles, light from the sensing area to direct the projected light toward the photoreceptor element; and a third deflecting member for directing the light, which has been introduced by the first deflecting member and has traversed the sensing area, toward the second deflecting member via the sensing area again.
Preferably, the third deflecting member includes two deflecting members for bending incident light approximately at right angles and directs light, which is approximately parallel to the light introduced to the sensing area by the first deflecting member, toward the second deflecting member.
Projected light from the light-projecting element is directed from the first deflecting member to the third deflecting member through the sensing area, advances from the third deflecting member to the second deflecting member through the sensing area again and arrives at the photoreceptor element.
The arrangement is such that the projected light traverses the sensing area at two or more times. As a result, if a transparent or semi-transparent body is present in the sensing area, the projected light passes through the transparent or semi-transparent body at least two times. Since the amount of attenuation of the projected light is increased, even a transparent body (or semi-transparent body) can be sensed with certainty. Since the arrangement is such that the projected light arrives at the photoreceptor element upon being deflected approximately at right angles by the first deflecting member and deflected approximately at right angles by the second deflecting member, the projected light does reach the photoreceptor element directly. This makes it possible to prevent erroneous detection due to reflection or the like.
The first type of transmission-type photoelectric sensor can be used in a multiple-type configuration as well. In such case it is preferred that the photoelectric sensor include at least two, namely first and second, sensing arms disposed so as to oppose each other across a sensing area defined therebetween. The first sensing arm has, at a distal end thereof, a first deflecting member for deflecting projected light from a light-projecting element, which projected light is introduced from a base end of the first sensing arm and advances along the longitudinal direction of the arm, in a direction approximately perpendicular to the longitudinal direction of the arm, thereby directing the light toward the sensing area; and, at the distal end thereof, a second deflecting member for deflecting light from the sensing area in a direction approximately at right angles, causing the light to advance along the longitudinal direction of the arm and introducing the light to the photoreceptor element. The second sensing arm has a third deflecting member for causing light, which has advanced from the first deflecting member via the sensing area, toward the second deflecting member via the sensing area again.
A photoelectric sensing method according to the present invention can be expressed as follows: Specifically, a photoelectric sensing method according to the present invention includes causing light from a light-projecting element to advance to a sensing area upon being deflected approximately at right angles, directing the light, which has traversed the sensing area, toward the sensing area again to thereby cause the light to traverse the sensing area at least two times, introducing the light, which has traversed the sensing area at least two times, to a photoreceptor element upon deflecting the light approximately at right angles, and sensing an object, which is present in the sensing area, based upon an output signal from the photoreceptor element.
A second type of transmission-type photoelectric sensor has at least first and second sensing arms disposed so as to oppose each other across a sensing area defined therebetween. The first sensing arm has, at a distal end thereof, a first deflecting member for deflecting projected light from a light-projecting element, which projected light is introduced from a base end of the first sensing arm and advances along the longitudinal direction of the arm, in a direction that is approximately perpendicular to the longitudinal direction of the arm and oblique with respect to a direction in which the first and second sensing arms are arrayed, thereby directing the light toward the sensing area. The second sensing arm has, at a distal end thereof, a second deflecting member for deflecting light, which has advanced from the first deflecting member of the first sensing arm obliquely through the sensing area, in an approximately perpendicular direction, causing the light to advance along the longitudinal direction of the arm and introducing the light to a photoreceptor element.
The arrangement is such that the projected light crosses the sensing area obliquely. The projected light impinges obliquely upon the surface of an object to be sensed. Since loss due to reflection of the obliquely incident light is large, the fact that the difference in amount of light incident upon the photoreceptor element when an object to be sensed is and is not present is large makes it possible to sense the object with certainty, even if the object to be sensed is transparent (or semi-transparent).
In a preferred embodiment, both the first sensing arm and the second sensing arm are provided with the first and second deflecting members.
Preferably, the front side of the photoreceptor element is provided with a slit which limits the incident light. The photoreceptor element may be a position sensing device.
A third type of transmission-type photoelectric sensor includes a first sensing arm, and second and third sensing arms disposed on respective ones of both sides of the first sensing arm across sensing areas defined therebetween. The first sensing arm has, at a distal end thereof, a first splitting and deflecting member for splitting, into two portions, projected light from a light-projecting element, which projected light is introduced from a base end of the first sensing arm and advances along the longitudinal direction of the arm, deflecting these split portions of light approximately at right angles, and directing these split portions of light toward the sensing areas on both sides. The second and third sensing arms respectively have, at respective distal ends thereof, second and third deflecting members, respectively, for deflecting light, which has advanced from the first splitting and deflecting member through the sensing area, approximately at right angles, causing the light to advance along the longitudinal direction of the arm and introducing the light to photoreceptor elements.
Projected light from one light-projecting element is split into two portions by the first splitting and deflecting member and advances into the two sensing areas. The two beams of light that have traversed these sensing areas are deflected by respective ones of the second and third deflecting members and are received by respective ones of separate photoreceptor elements. Sensing in regard to objects in two sensing areas is possible by a single light-projecting element, thereby reducing the number of elements. In a case where this photoelectric sensor is applied to a multiple-type configuration, the numbers of light-projecting elements and photoreceptor elements can be reduced greatly.